N = R L - University of Illinois at...
Transcript of N = R L - University of Illinois at...
Astronomy 330
This class (Lecture 7):
Making C,O, and N
Next Class:
Star Formation
HW 3 due Wednesday.
Music: Sonne– Rammstein
# of
advanced
civilizations
we can
contact in
our Galaxy
today
Drake Equation HW #1 Result
N = R* ! fp ! ne ! fl ! fi ! fc ! L Star
formation
rate
Fraction
of stars
with
planets
# of
Earthlike
planets
per
system
Fraction
on which
life arises
Fraction
that evolve
intelligence
Fraction
that
commun-
icate
Lifetime of
advanced
civilizations
stars/
yr
systems/
star
planets/
system
life/
planet
intel./
life
comm./
intel.
yrs/
comm.
Grouped
•! We spent last week discussing the End of the
Universe.
•! In groups of 3-4, try to write a 3-4 sentence
discussion of the End of the Universe for a non-
science major friend.
Outline
•! Sneaky little neutrinos (proof of fusion)
•! C and O for the first time (1st gen of stars)
•! N for the first time (2nd gen of stars)
•! Molecules are for lovers!
The Proton-Proton (p-p) Chain
Nuclear Fusion in the Sun’s
Interior
•! Proton-Proton Chain
–! 4 hydrogen atoms fuse
to make 1 helium atom
–! Requires very high
density and
temperature (at least 7
million K)
http://www.youtube.com/watch?v=Czbh_sdqX84
Nuclear Reactions in the Sun
•! Chain: 4 protons helium
•! First step in chain (2 protons combine):
•! Start with 2 particles (protons)
•! End up with 4 particles (two of which are glued
together)
•! Each product is very interesting in its own right....
Nuclear Reactions in the Sun
[np] = deuterium
•! 1 proton + 1 neutron bound together into nucleus of element…
•! Hydrogen, but has neutron, so 2 times mass of normal H
•! “Heavy Hydrogen”
•! Simplest composite nucleus
Discovery of D in lab: Nobel Prize
about 0.01% of all H on earth is D
!! including in your body:
you contain about 10 kilos (20 lbs) of H, and about 2 grams of D
!! Water (normally H2O) with D is D2O : “heavy water"
Nuclear Reactions in the Sun
e+ = positron •! Exactly the same as electron but charge +1
•! Antimatter
•! Combines with normal e-
–! Both are gone, release of energy
–! Annihilation
Discovery of positron in lab: Nobel Prize
Because of this reaction
"! The Sun contains a small amount of antimatter!
Nuclear Reactions in the Sun
"! (Greek letter “nu”) = neutrino
#! Particle produced in nuclear reactions only
#! Tiny mass: m(") < 10-6m(e) !
#! Moves at nearly the speed of light
#! Very weakly interacting
Discovery of neutrino in lab: Nobel Prize
10 billion from Sun go through hand every sec
"! Reach out!
"! Go through your body, Earth, but almost never interact
Why Doesn’t The Sun Shrink?
•! Sun is currently
stable
•! Pressure from
the radiation
created by
fusion balances
the force of
gravity.
•! Gravity is balanced
by pressure from fusion!
Alf Doesn’t Care?
•! A star in hydrostatic equilibrium will
not shrink or swell.
•! It will maintain constant size, density,
and temperature for more than a
million years!
•! At this point, the star is called a main
sequence star. –! MS is when a star burns H into He
•! If stars were not constant, what effect
would that have on life on orbiting
planets. Ultraviolet light variations?
http://alf.disim.com/photos/
photoposter.htm
Nuclear Fusion in the Sun’s Interior
The Proton-Proton Cycle
•! Proton-proton in stars like the Sun
•!Hydrogen fused to make helium
•! 0.7% of mass converted to energy
The Sun is a mass of incandescent gas A gigantic nuclear furnace Where hydrogen is built into helium At a temperature of millions of degrees
The Sun is hot, the Sun is not A place where we could live But here on Earth there'd be no life Without the light it gives
We need its light We need its heat The Sun light that we seek
The Sun light comes from our own Sun's atomic energy
The Sun is a mass of incandescent gas A gigantic nuclear furnace Where hydrogen is built into helium At a temperature of millions of degrees
The Sun is hot
The Sun is so hot that everything on it is a gas: Aluminum, Copper, Iron, and many others
They Might Be Giants Why Does The Sun Shine The Sun is large... If the sun were hollow, a million Earth's would fit inside
And yet, it is only a middle-sized star
The Sun is far away... About 93,000,000 miles away
And that's why it looks so small
But even when it's out of sight
The Sun shines night and day
We need its heat, we need its light
The Sun light that we seek
The Sun light comes from our own sun's atomic energy
Scientists have found that the Sun is a huge atom smashing machine
The heat and light of the sun are caused by nuclear reactions between Hydrogen, Nitrogen, Carbon, and Helium
The Sun is a mass of incandescent gas
A gigantic nuclear furnace
Where Hydrogen is built into Helium
At a temperature of millions of degrees
Why Nuclear Fusion Doesn’t Occur in Your Coffee
•! Fusion requires:
•!High enough temperature
(> 5 million K)
•!High enough density
•! Enough time
Sneaky Little Neutrinos
•! The Sun’s nuclear fusion produces a particle called a
neutrino
•! Matter is almost transparent to neutrinos
•! On average, it would take a block of lead over a quarter of
a light-year long to stop one
•! Roughly 1 billion pass through every square centimeter of
you every second!
Super Kamiokande, Japan
50,000 tons of water
Neutrinos
40 meters
Detecting Neutrinos
90 degrees
90
de
gre
es
The Sun in Neutrinos
•! Confirmation
that nuclear fusion
is happening in
the Sun’s core
•! 500 days of data
•! As they can only be
produced by nuclear
processes, our
energy source
concept must be
fundamental
Cosmic Gall NEUTRINOS, they are very small.
They have no charge and have no mass And do not interact at all. The earth is just a silly ball To them, through which they simply pass, Like dustmaids down a drafty hall Or photons through a sheet of glass. They snub the most exquisite gas, Ignore the most substantial wall, Cold shoulder steel and sounding brass, Insult the stallion in his stall, And scorning barriers of class, Infiltrate you and me! Like tall and painless guillotines, they fall Down through our heads into the grass. At night, they enter at Nepal and pierce the lover and his lass From underneath the bed-you call It wonderful; I call it crass.
- Telephone Poles and Other Poems, John Updike, Knopf, 1960
very little
X
X X hardly
Think-Pair-Share
If we could sustain fusion in the lab we could meet humankind’s energy needs nearly forever! Why is it so difficult to achieve this, when stars do it every day?
Important Questions
The Sun remains stable and on the main sequence as
long as it has hydrogen to fuse in the core… it will
evolve and will kill all life on Earth after all the fuel
is gone.
How long will the fuel last? What happens when
the fuel runs out?
How much Gas
do we have left?
•! Total energy available is easily calculated by mass
of hydrogen in Sun and energy released by each
hydrogen conversion.
•! We only have about 6 billion years left!
http://skeptically.org/sitebuildercontent/sitebuilderpictures/.pond/suv-econ-gas-pump.jpg.w300h294.jpg
Hungry, Hungry Sun
•! On the main sequence for ~11 billion years.
•! The core is where fusion occurs- H # He
•! Eventually, runs out of hydrogen in the core.
–! Rest of Sun is mostly hydrogen, but not in the core.
•! And it’s not hot enough to fuse helium!.....yet
The Battle between Gravity and Pressure
Gravity pushes in
Heat pressure
from H!He
fusion pushes
out.
Hydrostatic equilibrium: Balanced forces
The Battle between Gravity and Pressure
Gravity pushes in
With end of H
fusion, gravity
thinks it’s
winning
Unbalanced forces
The Red Giant Phase:
6 Billion Years •! When the hydrogen is gone
in the core, fusion stops
•! Core starts to contract
under its own gravity
•! This contracting heats
the core, and hydrogen
fusion starts in a shell
around the core
•! Energy is released, expands
envelope # brightness increases!
•! As the envelope expands, it cools – so it becomes a
red giant http://www.youtube.com/watch?
v=kWY7mS1A_AM&feature=fvw
Question
As the Sun moves off the main sequence what
happens in the core?
a)! Hydrogen burning stops
b)! Helium burning stops
c)! TNT burning stops
d)! We don’t know, but it makes the Sun red.
In 6-7 Billion years
•!The Sun will expand to
100-250 times bigger
than it is now!
•!The same mass but
now it’s bigger.
Contraction Junction
•! Core gets hotter, and hotter,
and hotter until…
•! 100 million degrees F
•! Core heats # He fusion
ignites
•! He # C & O
The Horizontal Branch
•! Helium burning stabilizes
the core
•! The outer envelope
shrinks, heats up, and
dims slightly
•! But helium doesn’t last
very long as a fuel
–! Horizontal branch lifetime
is only about 10% that of a
star’s main sequence lifetime
–! Our Sun will burn helium for about a billion years
–! Also He burning is unstable
When Helium Runs Out…
7.8 Billion Years
•! Fusion in the core stops – the helium has been converted to carbon and oxygen
•! Stellar core collapses under its own gravity
•! Shell starts fusing helium
•! Star starts to grow and cool again
•! Called an asymptotic giant branch star
Main sequence Core hydrogen burning Tcore ~ 16 million K
Red giant Shell hydrogen
burning
Asymptotic branch giant Shell helium burning
Helium flash
Life of a Low Mass Star
Horizontal branch Core helium burning Tcore ~ 100 million K
Question
As the Sun becomes an asymptotic giant branch star,
what is happening in the central core of the Sun?
a)! Hydrogen burning.
b)! Helium burning.
c)! TNT burning.
d)! Nothing is burning, fusion has stopped.
e)! We don’t know, but it makes the Sun red. NGC 2440
End Game
•! At these last stages, the Sun will likely oscillate in size and temperature.
•! This is messed up and creates a “Superwind”
•! Outer layers of the red giant star are cast off –! Up to 80% (at least 50%) of
the star’s original mass
NGC 2440
T > 200,000 K
End Game
•! “The core remains, made of carbon/oxygen “ash” from helium fusion
–! The core is very hot, above 200,000 K
•! Ultraviolet radiation from the core ionizes the cast off outer layers
–! Becomes a planetary nebula
–! Unfortunate name, but some of the most beautiful objects in the sky.
Planetary Nebulae
What About the Core?
•! Final fate - White dwarf –! Slowly cools off over
billions of years
–! Just a hot body
–! No fusion
–! Not really a star in some ways
–! Size of the Earth Sirius B
NGC 2440
T > 200,000 K
What Happens to Earth?
•! We have detected planets around white dwarfs, but they have presumably had a hard time.
•! If you were to visit the wasteland of Earth, the Sun would only be a very bright point of light.
•! Not sufficient for life.
Question
This is the way the Sun ends. This is the way the
Sun ends, not with a bang but a
a)! whimper; it just cools down over time.
b)! supernova blasting heavy elements into space.
c)! blackhole.
d)! planetary nebula and a white dwarf.
e)! a helium flash.
Nuclear Fusion in the First Stars
•! Core T > 10 million K
–! Violent collisions
–! e- stripped from atoms (ionized)
–! Nuclei collide, react
–!They get close enough that the nuclear strong force takes over.
•! Thru series (chain) of reactions
•! 4 protons helium (2p,2n) nucleus + energy
•! Fusion: light nuclei combine heavier nuclei
The First Stars
•! In the cores of the first stars, it gets hot enough for nuclear fusion.
•! In the internal furnace of these first stars is where carbon and
oxygen are created for the
first time in the Universe.
•! Higher density and
temperature of the
red giant phase
allows for the
creation of sulfur,
phosphorous,
silicon, and finally
iron.
Question
The rocky planets that formed around the first stars
would have been?
a)! A perfect place to raise a family.
b)! Devoid of the molecules necessary for life .
c)! Too close to the massive star to have life.
d)! Inhabited by truly alien creatures.
e)! Trick question. There would not have been any
rocky planets.
Stage Temperature Duration
H fusion 40 million K 7 million yr
He fusion 200 million K 500,000 yr
C fusion 600 million K 600 yr
Ne fusion 1.2 billion K 1 yr
O fusion 1.5 billion K 6 mo
Si fusion 2.7 billion K 1 day
Values for a 25MSun star
Iron – The End of the Road
•! “Burning” heavier and heavier atoms in the fusion process
•! Each stage faster than the last
•! After iron - no fuel left! –! It requires energy to produce
heavier atoms
Supernova!
http://www.spacetelescope.org/images/screen/heic0609c.jpg
C. Lithgow-Bertelloni (UM)
Making Heavy Elements
•! The star goes supernova and explodes. Some of C, O, P, S, Si, and Fe get carried away. At this point, even heavier elements can be made.
•! During the explosion, energy-consuming fusion reactions are possible
•! These by-products are blasted into space (>90% of star)
•! Supernovae provide much of the building blocks for planets… and us!
•! We are recycled supernova debris!
•! We are Star stuff.
Delenn, B5
CNO-ing
•! Now the Universe has some C and O laying
around; it can use it.
•! In the next generation of stars, the CNO cycle can
be used in the fusion process.
•! It is more efficient in stars slightly more massive
than the Sun.
•! Remember the Sun
mostly uses
proton-proton fusion.
Jan 31, 2008 Astronomy 330 Spring 2008
Hans Bethe
The CNO
Cycle
The Second Generation
•! The first stars blew up their new
elements into the proto-galaxy.
•! Now, the second stars form in the
ashes of the first.
•! With C and N, the 2nd generation
can form helium through the
CNO cycle, in which most of the
Universe’s nitrogen is created.
•! The 2nd generation also
eventually explodes blowing
nitrogen and the other elements
into the galaxy.
A supernova in a nearby galaxy. A
single star exploding can be brighter
than millions of stars in the nucleus.
The Next Stars
•! The new atomic elements from the 1st and 2nd stars are spread out into the galaxy.
•! The Sun must be at least a 3rd generation star as we have nitrogen in abundance.
•! Indeed, the percentage of heavier elements is larger toward the center of the galaxy, where the first generation of stars probably formed. (Seen in ours and other galaxies.)
•! Again, we are star stuff.
•! Keep in mind that this is all from the nuclear strong force– fusion.
The Chandra x-ray observatory has shown
that the CasA supernova has flung calcium,
iron, and silicon into space.
Question
HONC is important for life. In which order did these
elements first appear in the Universe?
a)! H, O, N, C
b)! All at once
c)! H, C, O, N
d)! N, O, H, C
e)! C, O, N, H